Metal cans are generally coated on interior and exterior surfaces thereof for prevention of corrosion. Recently, there has been promoted the development of methods for obtaining corrosion resistance without using an organic solvent for the purpose of simplifying production process, improving sanitation and preventing pollution. As one of the methods, coating of a metal can surface with a thermoplastic resin film has been attempted. That is, studies are under way on a method for making cans by laminating a thermoplastic resin film onto a plate of metal such as tin, tin-free steel, aluminum or the like and then, drawing the laminated metal plate. Although a polyolefin film or polyamide film has been tested as this thermoplastic resin film, neither of them satisfies all the requirements consisting of moldability, heat resistance, odor retainability and impact resistance.
In the prior art, a polyester film, particularly a polyethylene terephthalate film is paid much attention as a film having well-balanced properties and there have been made some proposals based on this film. However, polyethylene terephthalate films involve such a problem that moldability is poor when heat resistance and odor retainability are excellent, while odor retainability and retort resistance are poor when moldability is excellent. It has been becoming clear that a copolyester film is excellent in moldability, heat resistance, retort resistance and odor retainability and is suitable for making cans (see Japanese Laid-Open Patent Application 3-86729).
However, it has also been becoming clear that when this copolyester film is used in a drink can or food can, it has a problem in that it spoils the taste of the contents of the can due to its insufficient taste retainability.
Meanwhile, Japanese Laid-Open Patent Application 6-263893 discloses a polyester film to be laminated onto a metal plate, which contains 0.01 to 1 wt % of diethylene glycol to improve impact resistance and enumerates homopolymers and copolymers of polyethylene naphthalate as the polyester.
Japanese Laid-Open Patent Application 6-116486 discloses a copolyester film to be laminated onto a metal plate, which contains 0.05 to 20 wt % of polyoxyalkylene glycol (including diethylene glycol) to improve flavor-retention property and enumerates homopolymers and copolymers of polyethylene naphthalate as the polyester.
Japanese Laid-Open Patent Application 7-82391 discloses a biaxially oriented polyester film to be laminated onto a metal plate and molded, which is composed of a copolyester comprising 80 to 95 mol % of 2,6-naphthalenedicarboxylic acid and 5 to 20 mol % of an aliphatic dicarboxylic acid represented by the following formula: EQU HOCO.paren open-st.CH.sub.2 .paren close-st..sub.n --COOH
wherein n is an integer of 2 to 10, as acid components and ethylene glycol as a main glycol component, the copolyester having an intrinsic viscosity of 0.5 to 0.7 and containing a lubricant having an average particle diameter of 2.5 .mu.m or less.
It is an object of the present invention to provide a polyester film to be laminated onto a metal plate and molded, which has more excellent odor retainability and good retort resistance while retaining excellent properties of a conventional polyester film such as moldability, heat resistance, odor retainability and impact resistance.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention can be attained by a biaxially oriented polyester film to be laminated onto a metal plate and molded (may be referred to as "the first polyester film of the present invention" hereinafter):
(A) which comprises a copolyester comprising (a) 2,6-naphthalenedicarboxylic acid in an amount of 80 to 100 mol %, and either an aromatic dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid or a combination of the aromatic dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid and an aliphatic dicarboxylic acid in a amount of 20 to 0 mol % based on the whole dicarboxylic acid components, and (b) ethylene glycol in an amount of 72 to 99.95 mol %, diethylene glycol in an amount of 0.05 to 8 mol %, and a glycol other than ethylene glycol and diethylene glycol in an amount of 0 to 20 mol % based on the whole diol components, and having (c) a melting point of 198 to 268.degree. C.; PA1 (B) which has an extraction of 0.5 mg/inch.sup.2 (0.0775 mg/cm.sup.2) or less when subjected to an extraction treatment in ion exchange water at 120.degree. C. for 2 hours; and PA1 (C) which has a center line surface roughness (Ra) of the film surface in the range of 4 to 30 nm. PA1 (A) which comprises a blend of the first copolyester comprising (a) 2,6-naphthalenedicarboxylic acid in an amount of 80 to 100 mol % and a dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid in an amount of 20 to 0 mol % based on the whole dicarboxylic acid components, and (b) ethylene glycol in an amount of 72 to 99.95 mol % and a glycol other than ethylene glycol in an amount of 0.05 to 28 mol % based on the whole diol components, and having (c) a melting point of 198 to 268.degree. C., and the second copolyester comprising (a') terephthalic acid in an amount of 90 to 100 mol % and a dicarboxylic acid other than terephthalic acid in an amount of 10 to 0 mol % based on the whole dicarboxylic acid components, and (b') ethylene glycol in an amount of 90 to 99.95 mol % and a glycol other than diethylene glycol in an amount of 0.05 to 10 mol % based on the whole diol components, and having (c') a melting point of 230 to 255.degree. C., the content of diethylene glycol being 0.05 to 8 mol % based on the total mole number of the diol components of the first copolyester and the second copolyester; PA1 (B) which has an extraction of 0.5 mg/inch.sup.2 (0.0775 mg/cm.sup.2) or less when subjected to an extraction treatment in ion exchange water at 120.degree. C. for 2 hours; and PA1 (C) which has a center line surface roughness (Ra) of the film surface in the range 4 to 30 nm.
Secondly, the above objects and advantages can be attained by a biaxially oriented polyester film to be laminated onto a metal plate and molded (may be referred to as "the second polyester film of the present invention" hereinafter):
The polyester film of the present invention will be detailed below. A description is first given of the first polyester film.
The copolyester constituting the first copolyester film of the present invention comprises 2,6-naphthalenedicarboxylic acid in an amount of 80 to 100 mol % of the total of all dicarboxylic acid components and ethylene glycol in an amount of 72 to 99.95 mol % of the total of all diol components.
The remaining 0 to 20 mol % of the total of all dicarboxylic acid components comprises either an aromatic dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid or a combination of the aromatic dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid and an aliphatic dicarboxylic acid. Of the two, the aromatic dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid is preferred as the remaining dicarboxylic acid component.
Preferred examples of the aromatic dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid include phthalic acid, isophthalic acid, terephthalic acid, 2,7-naphthalenedicarboxylic acid and the like. Illustrative examples of the aliphatic dicarboxylic acid used in combination of these aromatic dicarboxylic acids include oxalic acid, adipic acid, succinic acid, sebacic acid, azalaic acid, decanedicarboxylic acid and the like. They may be used alone or in combination of two or more.
The remaining 0.5 to 28 mol % of the total of all diol components consists of 0.05 to 8 mol % of diethylene glycol and 0 to 20 mol % of a glycol other than ethylene glycol and diethylene glycol. The total of all diol components preferably consists of 82 to 99.95 mol % of ethylene glycol and 0.05 to 8 mol %, more preferably 0.1 to 5 mol %, particularly preferably 0.5 to 3 mol %, of diethylene glycol.
When the content of diethylene glycol is less than 0.05 mol %, the effect of improving odor retainability is insufficient. On the other hand, when it is more than 8 mol %, the melting point of the obtained copolyester becomes too low with the result of deterioration in its heat resistance.
Preferred examples of the glycol other than ethylene glycol and diethylene glycol include propylene glycol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, cyclohexane dimethanol and the like. They may be used alone or in combination of two or more.
A known method can be used to cause diethylene glycol to be contained in the polyester film. For instance, diethylene glycol may be added in advance to an acid component and/or a glycol component which are/is a starting material(s) for a polyethylene-2,6-naphthalene dicarboxylate copolymer or may be added in any stages during a polycondensation reaction. Since diethylene glycol is usually by-produced in the polycondensation of a polyester containing ethylene glycol as a glycol component, the amount of diethylene glycol can be set to a predetermined amount by controlling the amount of the by-produced diethylene glycol.
In the present invention, the above copolyester is preferably produced using a germanium compound as a polymerization catalyst.
As the germanium catalyst,
(1) amorphous germanium oxide, PA0 (2) fine crystalline germanium oxide, PA0 (3) a solution obtained by dissolving germanium oxide in a glycol in the presence of an alkali metal or an alkaline earth metal or a compound of these metals, or PA0 (4) a solution obtained by dissolving germanium oxide in water is used, for example.
The copolyester preferably contains germanium atoms derived from the above polymerization catalyst in an amount of 40 to 200 ppm, more preferably 60 to 150 ppm.
The above copolyester in the present invention has a melting point of 198 to 268.degree. C. The lowest limit of 198.degree. C. corresponds to the melting point of a copolyester in which the total of the amount of a dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid based on the whole dicarboxylic acid components and the amount of a glycol other than ethylene glycol based on the whole diol components is almost 28 mol %. The upper limit of 268.degree. C. is nearly equal to the melting point of a copolyester which comprises 2,6-naphthalenedicarboxylic acid as the only dicarboxylic acid component and ethylene glycol in an amount of 99.95 mol % and diethylene glycol in an amount of 0.05 mol % based on the whole diol components.
The copolyester preferably has an intrinsic viscosity (measured in o-chlorophenol at 35.degree. C.) of 0.45 to 0.80 dl/g. The copolyester having the intrinsic viscosity in the above range has good processability.
The copolyester may further contain an antioxidant, heat stabilizer, ultraviolet absorber, antistatic agent and the like.
The first polyester film of the present invention has an extraction of 0.5 mg/inch.sup.2 (0.0775 mg/cm.sup.2) or less when subjected to an extraction treatment in ion exchange water at 120.degree. C. for 2 hours.
When the extraction is more than 0.5 mg/inch.sup.2, not only does the taste retainability deteriorate but a weight reduction at the time of a retort treatment also becomes large with the result of a decline in retort resistance.
The extraction is preferably 0.3 mg/inch.sup.2 or less, more preferably 0.1 mg/inch.sup.2 or less. The extract is believed to be substantially composed of a polyester oligomer.
To reduce the extraction to 0.5 mg/inch.sup.2 or less, the amount of the oligomer contained in the copolyester may be reduced, or an unstretched film formed by melt extrusion may be stretched in a longitudinal direction in multiple stages. This multi-stage stretching in a longitudinal direction also has the effect of improving moldability for can making or deep drawing by lowering the surface orientation of the film.
The first polyester film of the present invention must have a surface roughness (Ra) of 4 to 30 nm, preferably 5 to 20 nm. The term "surface roughness (Ra)" means center line average roughness (JIS-B 0601) which will be described later.
Although it may seem odd that there is a correlation between the surface roughness of the polyester film and its taste retainability in a drink can or food can, it is presumed that the latent performance of the polyester film related to its taste retainability is very correlative to the specific range of its surface roughness at the time of preparing raw materials, stretching the film, laminating the film onto a metal plate and molding the film and that the surface roughness is actually related to the taste retainability. The latent performance, however, is still unknown at the moment. If the polyester film has a surface roughness (Ra) of 4 to 30 nm, it has excellent taste retainability to be used as a drink can or a food can.
In the polyester film of the present invention, it seems that when the surface roughness (Ra) is less than 4 nm, the handling property (winding property) of the film deteriorates, while when the surface roughness is more than 30 nm, the taste retainability of the film deteriorates (in other words, the taste of the contents of the can spoils.)
From another point of view, when the surface roughness (Ra) is less than 4 nm, its handling property (winding property) deteriorates while when the surface roughness (Ra) is more than 30 nm, pinholes readily form, thereby deteriorating odor retainability disadvantageously (that is, spoiling the taste of the contents of the can).
The average particle diameter and amount of a lubricant to be added to the copolyester are suitably selected in order to obtain the polyester film having the surface roughness (Ra) of 4 to 30 nm. It is desired to add a lubricant having an average particle diameter of 0.01 to 2 .mu.m, preferably 0.05 to 1.5 .mu.m, in an amount of 0.01 to 1 wt %. A mixture of lubricants may also be used that differ from one another in kind and average particle diameter.
Although the lubricant to be added to the copolyester may be either inorganic or organic, it is preferably inorganic. Illustrative examples of the inorganic lubricant include silica, alumina, titanium dioxide, calcium carbonate, barium sulfate and the like. Illustrative examples of the organic lubricant include silicone resin particles, cross-linked polystyrene particles and the like. A monodisperse lubricant having a particle diameter ratio (long diameter/short diameter) of 1.0 to 1.2 is particularly preferred from a viewpoint of pinhole resistance. Illustrative examples of the monodisperse lubricant include spherical silica, spherical silicone resin particles, spherical cross-linked polystyrene and the like.
The surface roughness (Ra) of the film is a center line average surface roughness obtained in accordance with JIS-B0601. A value (Ra:nm) given from the following equation is defined as film surface roughness when a portion of a measurement length L is taken from a film surface roughness curve in its center line direction, the center line of this portion is taken as the X axis, the direction of longitudinal magnification is taken as the Y axis and the roughness curve is expressed by Y=f(x). EQU Ra=1/L.intg..sub.0.sup.L .vertline.f(x).vertline.dx
In the present invention, the surface roughness of 5 portions having a reference length of 2.5 mm are measured and an average value of four measurement values, excluding the largest value from the five measurement values, is taken as Ra.
The first polyester film of the present invention is preferably stretched in a longitudinal direction in multiple stages to improve moldability as described above. By stretching the film in a longitudinal direction in multiple stages, the surface orientation of the film is lowered and the moldability thereof is improved.
A description is subsequently given of the second polyester film of the present invention.
The second polyester film of the present invention is composed of a blend of the first copolyester (to be referred to as "copolyester A" hereinafter) and the second copolyester (to be referred to as "copolyester B" hereinafter).
The copolyester A comprises 2,6-naphthalenedicarboxylic acid in an amount of 80 to 100 mol % of the total of all dicarboxylic acid components and a dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid in an amount of 20 to 0 mol %.
The dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid may be either an aromatic dicarboxylic acid or an aliphatic dicarboxylic acid. Illustrative examples of the aromatic dicarboxylic acid and the aliphatic dicarboxylic acid are the same as those listed for the description of the first polyester film. Of the two, the aromatic dicarboxylic acid is more preferred as the dicarboxylic acid other than 2,6-naphthalenedicarboxylic acid.
The copolyester A comprises ethylene glycol in an amount of 72 to 99.95 mol % of the total of all diol components and a glycol other than ethylene glycol in an amount of 0.05 to 28 mol %.
Illustrative examples of the glycol other than ethylene glycol include diethylene glycol and those listed for the first polyester film.
The diol components of the copolyester A preferably comprise ethylene glycol in an amount of 82 to 99.95 mol % and diethylene glycol in an amount of 0.05 to 8 mol %. The diethylene glycol is contained more preferably in an amount of 0.1 to 8 mol %, particularly preferably 0.3 to 3 mol %. The reason why the above proportion of diethylene glycol is preferable and how to introduce diethylene glycol have already been set forth in the description of the first polyester film.
The copolyester A preferably contains germanium atoms derived from the polymerization catalyst in an amount of 40 to 200 ppm, more preferably 60 to 150 ppm. The polymerization catalysts are the same as those given in the description of the first polyester film.
The copolyester A has a melting point of 198 to 268.degree. C. The lower limit and upper limit of the melting point correspond to those of the melting point of the copolyester A which comprises copolymer components other than 2,6-naphthalenedicarboxylic acid and ethylene glycol forming the copolyester A in an amount of 0.05 mol % and 28 mol %, respectively.
The copolyester A preferably has an intrinsic viscosity (measured in o-chlorophenol at 35.degree. C.) of 0.45 to 0.80 dl/g.
The copolyester B (the second copolyester) comprises terephthalic acid in an amount of 90 to 100 mol % of the total of all dicarboxylic acid components and a dicarboxylic acid other than terephthalic acid in an amount of 10 to 0 mol %.
The dicarboxylic acid other than terephthalic acid is preferably contained in an amount of 5 to 0 mol %.
Illustrative examples of the dicarboxylic acid other than terephthalic acid are the same as those listed for the description of 2,6-naphthalenedicarboxylic acid and the copolyester A.
The copolyester B comprises ethylene glycol in an amount of 90 to 99.95 mol % of the total of all diol components and a glycol other than ethylene glycol in an amount of 0.05 to 10 mol %. Illustrative examples of the glycol other than ethylene glycol are the same as those listed for the description of the copolyester A.
The glycol other than ethylene glycol is preferably 0.05 to 8 mol %, more preferably 0.1 to 5 mol %, particularly preferably 0.3 to 3 mol % of diethylene glycol.
The copolyester B preferably contains germanium atoms derived from the polymerization catalyst in an amount of 40 to 200 ppm, more preferably 60 to 150 ppm. The polymerization catalysts are the same as those mentioned in the description of the first polyester film.
The copolyester B has a melting point of 230 to 255.degree. C. The upper limit and lower limit of the melting point correspond to those of the melting point of the copolyester B which comprises copolymer components other than terephthalic acid and ethylene glycol forming the copolyester B in an amount of 0.05 mol % and 10 mol %, respectively. The copolyester B preferably has an intrinsic viscosity (measured in o-chlorophenol at 35.degree. C.) of 0.50 to 0.80 dl/g.
The blend of the copolyester A and the copolyester B contains diethylene glycol in an amount of 0.05 to 8 mol % based on the total mole number of the diol components of the copolyester A and the diol components of the copolyester B. The blend preferably contains diethylene glycol in an amount of 0.1 to 5 mol %, more preferably 0.3 to 3 mol % based on the same basis. Below 0.05 mol %, the effect of improving taste retainability is insufficient, while above 8 mol %, the melting point of the blend becomes too low with the result of deterioration of heat resistance.
As for the ratio of the copolyester A to the copolyester B of the blend, the copolyester A is contained in an amount of 70 wt % or more, preferably 75 to 95 wt % and the copolyester B is contained in an amount of 5 to 25 wt % based on the total weight of the copolyester A and the copolyester B.
Both the copolyester A and the copolyester B may contain an antioxidant, heat stabilizer, ultraviolet absorber, antistatic agent and the like.
The second polyester film of the present invention has an extraction of 0.5 mg/inch.sup.2 (0.0775 mg/cm.sup.2) or less when it is subjected to an extraction treatment in ion exchange water at 120.degree. C. for 2 hours, and has a center line surface roughness (Ra) of the film surface in the range of 4 to 30 nm, like the first polyester film. Therefore, as for items not described herein in connection with extraction and Ra, it should be understood that the description of the first polyester film can be directly applied to the second polyester film.
The above value of Ra can be achieved by incorporating fine particles (lubricant) having an average particle diameter of 0.01 to 2 .mu.m and a particle diameter ratio of 1.0 to 1.2 into at least one of the copolyester A and the copolyester B in an amount of 0.01 to 1 wt %. The extraction is preferably 0.3 mg/inch.sup.2 (0.0465 mg/cm.sup.2) or less and Ra is preferably in the range of 5 to 20 nm.
The second polyester film of the present invention is preferably stretched in a longitudinal direction in multiple stages to improve moldability. By stretching the film in a longitudinal direction in multiple stages, the surface orientation of the film is lowered and the moldability thereof is improved.
The first polyester film and the second polyester film of the present invention preferably have each a thickness of 6 to 75 .mu.m, more preferably 10 to 75 .mu.m, particularly preferably 15 to 50 .mu.m. The film having a thickness of less than 6 .mu.m is easily broken at the time of molding, while the film having a thickness of more than 75 .mu.m is of excessively high quality uneconomically.
A metal plate such as tin, tin-free steel, aluminum or the like is suitably used as a metal plate to be laminated with the polyester film of the present invention, particularly a metal plate for making cans.
The lamination of the polyester film on the metal plate can be carried out in accordance with the following methods (a) and (b).
(a) After the film is laminated on the metal plate heated at a temperature higher than the melting point of the film, the metal plate is quenched to make the surface layer portion (thin layer portion) of the film in contact with the metal plate amorphous and joined to the film.
(b) The film is coated in advance with a primer on its one surface to form an adhesive layer and laminated on the metal plate in such a manner that the surface is in contact with the metal plate. Known resin adhesives such as epoxy-based adhesives, epoxy-ester-based adhesives and alkyd-based adhesives can be used as the adhesive.